1. Field of the Invention
The present invention relates to a method of automatically correcting aberrations in an electron beam, a method of visualizing the aberrations, and an automated aberration corrector.
2. Description of Related Art
In SEM (scanning electron microscopy) imaging, there is the problem that various aberrations in the electron beam deteriorate the resolution. In an attempt to solve this problem, CEOS GmbH (Germany) has developed an aberration corrector for correcting aberrations in an electron beam by the use of electric and magnetic fields that are not rotationally symmetric.
One known instrument of this kind detects an image by an underfocused or overfocused beam, for example. The image is Fourier-transformed. Given computational processing is performed. Then, an inverse-Fourier transform is performed to detect the brightness contour of the probe (see Japan Unexamined Patent Publication No. 2003-521801, for example). Another known technique finds the autocorrelation function of the distribution of differential optical intensities by finding the distribution of optical intensities of an object to be observed placed on the optical axis of a lens and differentiating the distribution to find the distribution of the differentiated optical intensities (see Japan Patent Laid-Open No. H8-124838, for example).
In any of the above-described known instruments, however, it is necessary to complexly control voltages applied to plural polar elements to correct aberrations in the electron beam, the polar elements being equipped in the instrument. FIG. 8 is a diagram illustrating a method of operating an electrostatic multipolar unit 100 having 12 polar elements as an electrostatic multipolar unit having less than 12 polar elements. As shown in the figure, the electrostatic dodecapole unit 100 has polar elements U1 to U12 which are activated via driver amplifiers A1 to A12, respectively, incorporated inside a driver portion 9. The amplifiers A1 to A12 are connected with power supplies P1 to P12, respectively. In the case of a magnetic dodecapole unit, a similar configuration is adopted.
The electrostatic dodecapole unit 100 constructed in this way can produce, by its twelve polar elements, static electric field, dipolar electric field, quadrupole electric field, hexapole electric field, and octopole electric field. Aberrations in the electron beam are corrected by combining these fields. Accordingly, there is the problem that in order to correct aberrations in an electron beam, the operator needs to have skilled technique and fully understand the effects of these fields on the electron beam. An aberration corrector is made up of plural stages. This electrostatic 12-pole unit 100 forms one of the stages.
In order to correct the aberrations in an electron beam using equipment having such multiple polar elements as described above, it is necessary to complexly control the voltages applied to the polar elements. Attempts have been made to automate this operation. However, noise in the image may make it difficult to provide automated control. Manual scanning may result in faster operation if the operator has some degree of skillfulness. However, where the operator manually controls the aberration corrector, it is necessary to estimate the state of aberrations in the electron beam while watching the image. To perform this estimation, the operator needs extensive experience and skillfulness.
In the above-cited Japan Unexamined Patent Publication No. 2003-521801, only a method of measuring the aberrations in an electron microscope is described. Any structure for automatically correcting electron beam aberrations by providing feedback from a computer is not described. Furthermore, any procedure for previously performing correction of deviation of the axis of the electron beam and focusing is not described, though measurements of various aberrations in electron beams are set forth.